The present invention relates to an image-forming apparatus in which a plurality of light-sources expose a photosensitive material to form an image on it, and specifically relates to an image-forming apparatus, which can prevent unevenness of the image by controlling the brightness of light-sources disposed at the both ends of the array of the plurality of light-sources.
For instance, a color image-recording apparatus, which produces a color proof for confirming image data utilized for forming an original plate of printing matters, etc., can be cited as the image-forming apparatus, which forms an image by exposing the photosensitive material with the plurality of light-sources.
In such the image-forming apparatus, a plurality of light beams, each of which is emitted from each of light-emitting elements having colors different each other, such as Read, Green, Blue, etc., are synthesized into one light-beam, which is irradiated onto the photosensitive material. Light emitting diodes (hereinafter, referred to as LEDs) or semiconductor laser devices, etc., are employed as the light-emitting elements mentioned above, and the color image is formed by exposing the photosensitive material with the light beams irradiated from such the light-emitting elements, the light amounts of which are modulated in stepwise on the basis of image data.
Incidentally, one of a plurality of light sources, into each of which light beams emitted from a plurality of light-emitting elements are synthesized, is defined as one channel. The image-forming apparatus incorporates an optical unit in which a plurality of channels are arranged in a sub-scanning direction. Further, the optical unit exposes the photosensitive material, which is rotating in a main-scanning direction, by irradiating the light beams onto it to form the image.
In the abovementioned operation, for every one-revolution of the photosensitive material in the main-scanning direction, the optical unit moves a distance equivalent to the width of the light beam in the sub-scanning direction. Then, the light beams are irradiated onto next recording area to continuously conduct the image-exposing operation. Otherwise, it is applicable that movements in both main-scanning and sub-scanning direction are combined relative to each other. In this case, the optical unit spirally moves on the photosensitive material. Incidentally, a part of the photosensitive material, on which an image-exposing operation with the light beams is conducted during one main-scanning operation, is defined as a recording area.
Incidentally, a light beam, into which light beams emitted from a plurality of light-emitting elements, having colors different each other, are synthesized, is defined as one channel. In the image-forming apparatus having a plurality of channels, there has been a problem that, since the beam profiles vary depending on mounting positions of the light-emitting elements in each channel and manufacturing errors, in angle, etc., of the light-emitting elements itself, image-unevenness are liable to occur when forming the image by irradiating the light beams onto the photosensitive material. To solve the abovementioned problem, each of the beam profiles emitted by each of the light-emitting elements has been adjusted by means of compensation lenses or position/angle adjusting functions, etc., provided for all of the light-emitting elements.
To adjust the beam profiles of the light-emitting elements by means of compensation lenses, etc., however, the cost for employing the compensation lenses are incurred as an additional cost, resulting in an increase of the manufacturing cost of the image-forming apparatus itself. In addition, it has been very difficult to uniformly adjust the all of the beam profiles of the light-emitting elements by means of compensation lenses, etc., and it has been a problem that adjusting processes of the compensation lenses, etc., have considerably increase the total number of the manufacturing processes of the image-forming apparatus.
Further, in the image-forming apparatus having a plurality of channels, it has been a problem that image-unevenness are generated in the image formed on the photosensitive material, due to material-characteristics of the photosensitive material and manufacturing variations occurred during the assembling process of the optical unit in the factory.
Section (a) of FIG. 11 shows a graph of distributions of the light beams irradiated from “n” channels of the optical unit in respect to the positional coordinate of each of “n” channels. The beam profiles irradiated onto the exposing surface of the photosensitive material are indicated at the upper section of FIG. 11. In the beam profiles shown in FIG. 11, the center areas are denoted by dot 100-1, 100-2, - - - , 100-n (hereinafter, totally denoted by “dots 100”). Concretely speaking, dot 100-1 denotes the center area of the beam profile emitted from channel 1, while dot 100-n denotes the center area of the beam profile emitted from channel “n”.
Further, in the beam profiles shown in FIG. 11, the peripheral areas dispersed from dots 100 are denoted by peripheral area 101-1, 101-2, - - - , 101-n (hereinafter, totally denoted by “peripheral areas 101”). Still further, in section (a) of FIG. 11, waveforms N1, N2, - - - , Nn indicate light amount characteristics of channels 1, 2, - - - , “n”, respectively.
In section (a) of FIG. 11, each of peripheral areas 101 of the light beams irradiated from channels 2, 3, - - - , “n-1” (hereinafter, referred to as central arrayed channels), excluding channels 1 and “n” (hereinafter, referred to as both-end channels) from “n” channels, overlaps with those of adjacent light beams located at both sides of it. Accordingly, the light amount of the light beams, irradiated onto the exposing surface of the photosensitive material from the central arrayed channels, becomes uniform.
With respect to the both-end channels, however, there is no adjacent light beam at one side of it. Therefore, the light amount, irradiated onto the exposing surface of the photosensitive material from the both-end channels, abruptly decreases, as shown in section (a) of FIG. 11.
Section (b) of FIG. 11 shows a graph of color developing density of the photosensitive material exposed by the light beams irradiated from the optical unit in respect to the positional coordinate of each of “n” channels. In section (b) of FIG. 11, the solid line indicates the density characteristics of material “a”, while the one dotted chain line indicates the density characteristics of material “b”. Materials “a” and “b” are photosensitive materials having material characteristics being different each other.
Further, FIG. 12 shows the material characteristics of materials “a” and “b”. Density X indicates an optimum density for forming an image on the photosensitive material. When a light beam having light amount Y is exposed on materials “a” and “b”, a color of density X, being an optimum density, is developed on both materials. However, when the light amount of the light beam decreases to light amount Y′, which is exposed on materials “a” and “b”, a color of density XA is developed on material “a”, while a color of density XB is developed on material “b”.
As mentioned in the above, sometimes, the densities of the developed color are different relative to each other depending on the difference between material characteristics of the photosensitive materials. Accordingly, as shown in sections (a) and (b) of FIG. 11, the densities of the images formed at both ends of the recording areas of materials “a” and “b” are different relative to each other, due to the decrease of the light amount exposed by the light beam. Thus, it has been a problem that density-unevenness, generated in the formed image, have degraded the final image quality.
As a countermeasure to solve the above-mentioned problem, for instance, assuming that sections (a) of FIG. 11 indicates light amount characteristics of “n” channels at “m” times of main-scanning operations, it is applicable to control the movement of the optical unit in the sub-scanning direction so that peripheral area 101-n at “m” times of main-scanning operations overlaps with peripheral area 101-1 at “m+1” times of main-scanning operations, in order to compensate the decrease of the light amount of peripheral area 101-n. It is necessary, however, to vary a moving width in the sub-scanning direction corresponding to the material characteristics of the photosensitive material. Further, since color developing characteristics with respect to the light amount of the light-emitting element for each color are sometimes different respectively, and further, variations of the moving width in the sub-scanning direction result in a change of printed image size, it has been difficult to practically apply the abovementioned countermeasure.
As another countermeasure, it might be possible to adjust pitch “p” so that peripheral area 101-n at “m” times of main-scanning operations overlaps with peripheral area 101-1 at “m+1” times of main-scanning operations. As well as the above countermeasure, however, it has been difficult to vary pitch “p” corresponding to the difference between the material characteristics of the photosensitive material.
Further, sometimes, the light amount characteristics of the light beams emitted by the optical unit vary depending on the individuality of each optical unit, due to manufacturing variations occurred during the assembling process of the optical unit in the factory. In such the case, it is necessary to further apply the abovementioned countermeasure corresponding to the light amount characteristics of the individual optical unit, and it has been further difficult to practically apply the abovementioned countermeasure.
For instance, a method for compensating the light amount of the light beams irradiated from both-end channels in response to a moving amount in the sub-scanning direction is set forth in Tokkaihei 10-181085 and is well-known as one of conventional technologies. This method, however, requires a detecting circuit for detecting the moving amount in the sub-scanning direction, resulting in a complicated configuration of the image-forming apparatus. Further, a method for changing an intensity of the laser beam with the compensating laser beams equipped for both-end channels in response to image data is set forth in Tokkaihei 6-198952 and is well-known as another one of conventional technologies. However, when the intensities of the laser beams irradiated from both-end channels are too strong, it is impossible to subtract the intensity of the laser beam with the compensating laser beams.
Next, when the image is formed on the photosensitive material by simultaneously scanning a plurality of light beams along a plurality of lines on the photosensitive material in a main-scanning direction while moving them in a sub-scanning direction, the image-unevenness would be generated due to moving errors in the sub-scanning direction.
The image-unevenness would occur, when main-scanning line A overlaps with next main-scanning line B as shown in FIG. 24(a), or when a gap is generated between main-scanning line A and next main-scanning line B as shown in FIG. 24(b).
In case shown in FIG. 24(a), the density of overlapped area C, which is higher than the objective density for the area, periodically emerges as shown in FIG. 24(c) while the density of gap area D, which is lower than the objective density for the area, periodically emerges as shown in FIG. 24(d), resulting in the image-unevenness.
To eliminate such the image-unevenness, the following methods have been proposed.    (1) As set forth in Tokkaihei 10-181085, by detecting the moving amount in the sub-scanning direction, the image-forming operation is conducted in such a manner that, when two main-scanning lines overlap each other, the density of the overlapped area within the next main-scanning line is lowered, while, when the gap is generated between two main-scanning lines, intensity of the light beam irradiated onto a part adjacent to the gap is increased.    (2) As set forth in Tokkaihei 6-198952, by providing the compensating laser beam for irradiating a light onto the gap portion, when the gap is generated between two main-scanning lines, intensity of the light beam irradiated by the compensating laser beam is changed corresponding to the gap.
The methods described in items (1) and (2), however, have included the following problems.
Regarding to item (1), to accurately detect the moving amount in the sub-scanning direction, the additional devices, such as a liner encoder, etc., are necessary, resulting in a big raise of the manufacturing cost.
Regarding to item (2), the method is effective only for compensating the gaps generated between main-scanning lines.    (3) The abovementioned image-unevenness can be categorized into three cases, such as the first case in which a main-scanning line overlaps with the next main-scanning line, the second case in which a gap is generated between a main-scanning line and the next main-scanning line and the third case in which either a main-scanning line overlaps with the next main-scanning line or a gap is generated between a main-scanning line and the next main-scanning line. However, one of the three cases is automatically determined for each of the apparatus.    (4) As shown in FIG. 25, the image-unevenness becomes unnoticeable at a certain number of unevenness bars per unit area, depending on the density difference.